The present invention relates to a toner for use in a developer for developing an electrostatic image in electrophotography, electrostatic recording, electrostatic printing and so on, and an image forming method using the toner, and more particularly, to a dry toner for use in an image forming apparatus, such as a copying machine, a laser printer or a plain paper facsimile machine, and an image forming method using the toner. Moreover, the present invention also relates to a dry toner for use in a full-color copying machine, a full-color laser printer and a full-color plain paper facsimile machine or the like image forming apparatus, and an image forming method using the toner.
A developer for use in electrophotography, electrostatic recording, electrostatic printing and so on is once adhered to an image carrier such as a photoconductor on which an electrostatic image has been formed in a developing process, then transferred from the photoconductor to a transfer medium such as a transfer paper in a transfer process, and fixed on the paper in a fixing process. As a developer for developing the electrostatic image formed on a latent image holding surface of the image carrier, a two-component developer comprising a carrier and a toner and a one-component developer requiring no carrier (magnetic or nonmagnetic toner) are known.
As a dry toner for use in electrophotography, electrostatic recording, electrostatic printing and so on, a toner obtained by melt-kneading a toner binder such as a styrene resin or a polyester together with a colorant and so on and finely pulverizing the kneaded mixture is conventionally used.
After having been developed and transferred to a paper or the like, such a dry toner is heat-melted and fixed with a heat roll. At this time, when the temperature of the heat roll is excessively high, the toner is excessively melted and adhered to the heat roll (hot offset). When the temperature of the heat roll is excessively low, the toner is not sufficiently melted, resulting in insufficient fixation. With a view to energy saving and downsizing of an apparatus such as a copying machine, a toner which does not cause hot offset at a high fixing temperature (namely, has hot offset resistance) and which can be fixed at a low fixing temperature (namely, has low temperature fixability) is demanded. The toner should also have heat-resistant preservability so as not to cause blocking during storage or under ambient temperature in an apparatus in which the toner is used. Especially, a toner for use in a full-color copying machine and a full-color printer need to have a low melt viscosity to provide gloss and color mixability in a printed image, so that a polyester type toner binder having a sharp melt property is used therein. Since such a toner is likely to cause hot offset, a silicone oil or the like is conventionally applied to a heat roll in full-color machines. However, in order to apply a silicone oil to a heat roll, an oil tank and an oil applying unit are necessary, which makes the apparatus unavoidably complicated and large. Also, application of oil causes deterioration of the heat roll, so that the heat roll requires regular maintenance. Additionally, it is unavoidable for the oil to adhere a copying paper and an OHP (overhead projector) film. Especially, the oil adhered to OHP film impairs color tone of a printed image.
For the purpose of producing an image with high fineness and high quality, improved toners having a small particle size or a narrow particle size distribution have been proposed. However, particles of a toner produced by a normal kneading-pulverizing method have irregular shapes. Thus, the toner particles are further pulverized to generate superfine particles or a fluidizing agent is buried in the surface of the toner particles when the toner is agitated with a carrier in a developing unit or when, in the case of being used as a one-component developer, the toner particles receive a contact stress from a developing roller, a toner supply roller, a layer thickness regulating blade, a frictional electrification blade and so on, resulting in deterioration of image quality. Also, the toner is poor in fluidity as a powder because of the irregular shapes of the particles thereof, and thus requires a large amount of fluidizing agent or cannot be filled in a toner bottle with a high filling rate, which hinders downsizing of the apparatus.
Additionally, a process of transferring an image formed of color toners to produce a full-color image from a photoconductor to a transfer medium or a paper is becoming more complicated, so that low transferability of a pulverized toner due to the irregular shapes of the particles thereof causes a void in a transfer image and an increase in consumption of toners to prevent it.
Thus, there is an increasing demand for reducing toner consumption without causing a void in a transferred image by improving transfer efficiency and for decreasing running cost. When transfer efficiency is significantly high, there is no need for a cleaning unit for removing untransferred toner from a photoconductor and a transfer medium, which leads to downsizing of the apparatus and cost reduction in manufacturing the same. This has also a merit of generating no waste toner. For the purpose of overcoming the drawbacks of the toner of irregular particle shape, there has been proposed various methods for producing spherical toner particles.
For the purpose of providing a toner having heat-resistant preservability, low-temperature fixability and hot offset resistance, there have been proposed (1) a toner in which a polyester partially crosslinked using a multifunctional monomer is used as a toner binder (Japanese Laid-Open Patent Publication No. S57-109825) and (2) a toner in which a urethane-modified polyester is used as a toner binder (Japanese Examined Patent Publication No. H07-101318). For the purpose of providing a toner for use in a full-color system which can reduce the amount of oil to be applied to the heat roll, (3) a toner produced by granulating polyester fine particles and wax fine particles is proposed (Japanese Laid-Open Patent Publication No. H07-56390). Proposed for the purpose of providing a toner having improved powder fluidity and transferability when its particle size is reduced are (4) a polymerized toner obtained by dispersing a vinyl monomer composition containing a colorant, a polar resin and a releasing agent in water and suspension-polymerizing the vinyl monomer composition (Japanese Laid-Open Patent Publication No. H09-43909) and (5) a toner obtained by sphering toner particles comprising a polyester type resin in water using a solvent (Japanese Laid-Open Patent Publication No. H09-34167).
Additionally, Japanese Laid-Open Patent Publication No. H11-133666 discloses a dry toner consisting of nearly spherical particles in which a polyester modified with urea a bond is used.
However, none of the toners (1) to (3) have sufficient powder fluidity and transferability and thus can produce a high-quality image even when its particle size is reduced. The toners (1) and (2) cannot compatibly satisfy the heat-resistant preservability and the low temperature fixability and do not develop sufficient gloss to be used in a full color system. The toner (3) is insufficient in the low-temperature fixability and the hot offset resistance in oilless fixation. The toners (4) and (5) are improved in the powder fluidity and the transferability. However, the toner (4) is insufficient in the low-temperature fixability and requires much energy to fix. This problem is pronounced when the toner is used in full-color printing. The toner (5), which is superior to the toner (4) in the low-temperature fixability, is insufficient in hot offset resistance and thus cannot preclude the necessity of the application of oil to the heat roll in a full-color system.
The toner (6) is excellent in that the viscoelasticity of the toner can be appropriately adjusted by using a polyester extended by a urea bond and that it can compatibly satisfy gloss and releasing property as a full-color toner. Especially, a phenomenon in which a fixing roller is electrified in use and unfixed toner on a transfer medium is electrostatically scattered or adhered to the fixing roller, namely, electrostatic offset can be reduced by neutralization between positive chargeability of the urea bond component and weakly negative chargeability of the polyester resin. However, the molecular weight distribution of the urea-extended polyester is not controlled and an appropriate molecular weight distribution to satisfy releasing property and gloss/transparency which conflict with each other in a color toner in oilless fixation has not been found.
It is, therefore, an object of the present invention to provide a dry toner which is excellent in powder fluidity and transferability when its particle size is reduced and in heat-resistant preservability, low-temperature fixability and hot offset resistance.
Another object of the present invention is to provide a dry toner which can produce high gloss in a printed image and does not require application of oil to a heat roll when used in a full-color copying machine or the like.
It is a further object of the present invention to provide an image forming method using the above dry toner.
As a result of earnest studies for solving the above problems, the present inventors have made the present invention.
In accordance with the present invention, there is provided a dry toner for developing an electrostatic image, comprising a toner binder comprising a modified polyester having such a molecular weight distribution according to gel permeation chromatography that (a) a main peak is present in a molecular weight region of 1000 to 30,000, (b) that portion of the modified polyester having a molecular weight of at least 30,000 accounts for 1 to 10% based on a total weight of the modified polyester and (c) a ratio (Mw/Mn) of the weight average molecular weight Mw of the modified polyester to the number average molecular weight Mn of the modified polyester is not smaller than 2 but not greater than 15.
In another aspect, the present invention provides a dry toner for developing an electrostatic image having a melt viscosity at 110xc2x0 C. of 2.0xc3x97103 to 2.0xc3x97104 Paxc2x7s and a melt viscosity at 130xc2x0 C. of 2.0xc3x97103 or less and providing such a fixed image on an overhead projector sheet that has a deposition amount of 0.8-1.2 mg/cm2 and has a contact angle to water of 90xc2x0-130xc2x0.